A common method to treat cancer is to give radiation or chemicals to damage the cancer cell's chromosomes (DNA) so badly that the cell dies. These treatments are, however, equally toxic to cells growing normally. It is difficult or impossible under most circumstances to limit a patient's exposure to the DNA damaging agent to only the cancer cells. One way to protect normally growing cells from the toxic anti-cancer treatment would be to simultaneously treat the cancer cells with a second agent whose activity reduces the amount of toxic radiation or chemical needed to be effective. This could be done by “sensitizing” the cancer to the toxic treatments so the growing tumor cells die when a smaller amount of toxic radiation or chemical is administered.
One way to “sensitize” an undesirable growing cell (such as a cancer cell) to a DNA-damaging agent is to manipulate the cell so it will die when it incurs less DNA damage. A proliferating cell can detect if and how much chromosome damage it has. If there is enough DNA damage, a normal cell commits suicide, killing itself by a preprogrammed cell death mechanism called “apoptosis.” This suicidal reaction is a protective mechanism because damaged DNA can result in mutation of genes and disease, such as cancer (Hartwell (1994) Science 266:1821–1828).
A damaged cell, however, actively tries to avoid suicide by repairing the chromosomal damage inflicted by the toxic treatments. By using its DNA repair machinery, the cell can save itself from what otherwise would be a suicide-initiating dose of radiation or base-damaging agent (Smith (1996) Mutation Research 340:109–124).
One avenue by which a cell can repair enough DNA damage to “save itself” from a suicidal fate takes advantage of the fact that the programmed cell death signal is triggered only at the stage of the cell's growth where its chromosomes begin to divide (called “mitosis”). If the cell has the time to repair enough damaged DNA before entry into mitosis it can avoid suicidal apoptosis. To gain sufficient repair time after detecting that its chromosomes have been damaged, the cell “stalls” its cell cycle at a stage just before entry into mitosis. If the damage is too great, not enough repair can be done and the cell dies (Paulovich (1997) Cell 88:315–321; O'Connor (1997) Cancer Surveys 29:151–182). Under “normal” circumstances, with an intact DNA repair and cell cycle “stalling” mechanisms, relatively larger of amounts of toxic agent must be administered to kill the growing cell. However, if this stalling mechanism can be disturbed, less DNA repair time is available, less DNA damage is fixed before entry into mitosis, and thus relatively lesser amounts of anti-cancer agent are needed to kill the cell. Thus, discovery of inhibitors of this stalling mechanism, and their co-administration with DNA base-damaging agents, would provide novel means of effectively treating cancer with lower doses of toxic agents.
G2/M checkpoints prevent the segregation of damaged chromosomes, which is likely to be important in human tumorigenesis. (Hartwell, L. H. and M. B. Kastan, Science 266:1821–1828 (1994); Paulovich, A. G., et al. Cell 88:315–321 (1997).) The transition from G2 to M is regulated, in part, by the G2-specific kinase consisting of Cdc2 and cyclin B1. (Nurse, P., Cell 79:547–550 (1994).) Many G2/M regulatory genes have been identified recently, such as Chk1, Chk2, ATM (MEC1 and TEL1 in S. cerevisiae and RAD3 in S. pombe) and the 14-3-3 family. (Agarwal, M. L., et al., Proc. Natl. Acad. Sci. USA 92:8493–8497 (1995); Elledge, S. J., Science 274:1664–1672 (1996); Morrow, D. M., et al., Cell 82:831–840 (1995); Paulovich, A. G., et al. supra; Savitsky, K., et al., Science 268:1749–1753 (1995); Weinert, T. A., et al., Genes Dev. 8:652–665 (1994).) Their products alter Cdc2 activity by inhibiting dephosphorylation of inhibitory sites on Cdc25C. p53 also has been implicated in an IR-induced G2/M checkpoint. (Agarwal, M. L., et al., supra; Bunz, F., A., et al., Science 282:1497–1501 (1998); Guillouf, C., et al., Oncogene 10:2263–2270 (1995); Powell, S. N., et al., Cancer Res 55:1643–1648 (1995); Stewart, N., et al., Oncogene 10:109–115 (1995).) It may modulate the G2/M transition by upregulating 14-3-3σ (12) and/or p21wafl. (Bunz, F., A., et al., Science 282:1497–1501 (1998); Dulic, V., et al., Mol. Cell Biol 18:546–557 (1998); Medema, R. H., et al., 16:431–441 (1998).) Consequently, cells lacking p53 show chromosome instability (Fukasawa, K., et al., Science 271:1744–1747 (1996)), a phenotype likely resulting from defects in the G2/M checkpoint. Therefore, a multiplicity of G2/M checkpoints in response to DNA damage may well involve redundant controls involving both p53-independent and p53-dependent pathways.
GADD45 is a 165-amino acid nuclear protein whose expression also is p53-dependent (Zhan, Q., et al., Mol. Cell Biol 13:4242–4250 (1993)). GADD45 was originally identified on the basis of a rapid induction in Chinese hamster ovarian cells after UV irradiation (Fornace, A. J., Jr., et al. Mol. Cell Biol 9:4196–4203 (1989).). Induction of GADD45 also was observed following treatment with many other types of DNA-damaging agents, including various environmental stresses, hypoxia, IR, genotoxic drugs and growth factor withdrawal. (Papathanasiou, M. A., et al., Mol. Cell Biol 11:1009–1016 (1991)). In mammalian cells, two additional family members with extensive sequence homology, GADD45β and GADD45γ, were identified recently. (Takekawa, M. and H. Saito, Cell 95:521–530 (1998).) Similar to p53-deficient cells, cells from Gadd45-deficient mice also show genomic instability, including chromosome abnormalities and centrosome amplification. (Hollander, M. C., et al., Nat. Genet 23:176–184 (1999).) It is known that GADD45 binds to PCNA, p21wafl and Cdc2. (Kearsey, J. M., et al., Oncogene 11:1675–1683 (1995); Smith, M. L., et al., Science 266:1376–1380 (1994); Zhan, Q., et al., Oncogene 18:2892–2900 (1999).) GADD45 has no inhibitory effect on the kinase activity of the G1-specific Cdk2/cyclin E complex. (Smith, M. L., et al., Science 266:1376–1380 (1994); Zhan, Q., et al., Oncogene 18:2892–2900 (1999).) Increased expression of GADD45 in primary human fibroblasts arrests cells at the G2-M boundary. This arrest was attenuated by the overexpression of cyclin B1 and Cdc25C.
Thus, GADD45 is a mediator of the G2-M stalling mechanism. It has been reported that blocking GADD45 expression by constitutive antisense oligonucleotide expression “sensitized” a human colon carcinoma cell line to killing by UV irradiation and by cisplatin, a DNA-damaging cancer chemotherapy drug (Smith (1996) Oncogene 13:2255–2263). Identification of novel modulators, particularly inhibitors, of GADD45 polypeptide activity would provide new means to inhibit proliferation of cancer cells. The invention fills these, and other, needs.